The ‘Impossible’ Australian Tank That Fired Two Howitzers To Test A Single British Gun D
October 1942, Fort Gellibrand, Williamstown, near Melbourne. Australian engineers wheeled out a tank with two cannons jammed side by side into a single turret. Both were 25-pounder gun howitzers. Both were rigged to fire at exactly the same instant. There was no room left for a crew.
The trigger was a lanyard pulled from outside the tank. This was not a battle tank. It was not a gimmick. It was an instrument. The engineers needed to know whether their new turret could survive the recoil of the British 17-pounder, the most powerful Allied tank gun then under development. Australia did not own a 17-pounder, so they simulated one.
Two 25-pounders fired together produced roughly 120% of a single 17-pounder’s recoil force. If the tank held, the next tank would carry the real gun. The lanyard was pulled. The tank held. Australia entered the Second World War with 14 tanks, four Vickers Medium Mark II specials from the interwar period, and 10 Vickers Light Mark VI delivered in 1939.
There was no domestic tank industry. There were no military-grade armor plate mills. There was no tank engine production. The country had never built a tank. Then France fell. On June 12th, 1940, Prime Minister Robert Menzies, Director-General of Munitions Essington Lewis, and Lieutenant-General Sir Brudenell White met in Canberra.
According to Yuri Pasholok’s research published in Tank Archives, the strategic situation was clear. After the defeat in France and the start of fighting in North Africa, Australia was no longer a British priority. Australia would have to build its own tanks. The order placed that day called for 859 cruiser tanks.
Britain dispatched Colonel W. D. Watson of the Royal Artillery in October 1940. He traveled via the United States studying American mass production methods alongside Australian engineer Alan Chamberlain. He landed in Australia in late December and was made director of tank design within the Directorate of Mechanization.
The General Staff specification arrived in November. 65 mm of frontal armor, 35 mph, 150-mile range, a 2-pounder main gun, and critically a maximum width of 9 ft 4 in dictated by the Australian railway loading gauge. Manufacturing was scattered across organizations that had never built a tank.
The New South Wales Government Railways Chullora tank assembly shops handled final assembly. Bradford and Kendall in Sydney cast the hulls. McKay Massey Harris, a tractor maker, built the running gear. General Motors Holden in Melbourne assembled the engine package. The result was the AC1 Sentinel.
It was an extraordinary engineering achievement. 28 tons of cast hull and turret as single pieces made from an experimental nickel-free alloy that substituted zirconium because nickel was rationed for naval armor. According to Tank Encyclopedia, no other country could cast a tank hull at this scale as a single piece in 1942.
65 AC1s were built between August 1942 and June 1943, but the gun was wrong. The 2-pounder, useful in 1940, was obsolete by the time the first Sentinel rolled off the line. Australian crews called it a popgun. Britain had no 6-pounders to spare. The hull was excellent. The gun killed it. Watson’s solution did not come from Britain. It came from inside Australia.
The Maribyrnong Ordnance Factory in Melbourne was already producing the Ordnance QF 25-pounder, an 87.6 mm gun howitzer. By the end of 1943, 1,527 would be built. It was the only credible Australian-made gun in the right caliber range. In December 1941, Watson sketched the first 25-pounder tank mount designs.
Two test guns arrived in February 1942. The second AC1 prototype was fitted with a new turret housing the 25-pounder and successfully test-fired on June 29th, 1942. Field Marshal Sir Thomas Blamey took the proposal to the War Cabinet in July. The AC3 Thunderbolt program was authorized.
200 tanks initially, 510 projected over the full production run. The choice of the 25-pounder is widely misunderstood. Tank Encyclopedia is explicit on this point. The gun was selected not for use as a close support weapon, contrary to popular belief, but for the value of its 20-pound armor-piercing shot and its high explosive shell.
Armor-piercing sat between the 2-pounder and the 6-pounder, adequate against Panzer III and early Panzer IV armor at typical engagement ranges, but the high explosive shell weighed 11.34 kg with a substantial bursting charge. It vastly outperformed the 6-pounder, which had no useful HE round, and the American 75 mm M3.
The AC3 was the only tank ever built that mounted the 25-pounder as primary armament. A persistent misconception calls the AC3’s gun a short howitzer. This is wrong. The AC3 carried a lengthened 25-pounder. The barrel was extended by 18 in from 28 calibers to roughly 32. Only the recoil mechanism was shortened from a 40-in stroke to 20 in with a compact recuperator mounted above the barrel in a distinctive bulge in the mantlet.
The famous short 25-pounder pack howitzer used later in New Guinea inherited the tank’s recoil system, but was a separate, genuinely shorter weapon developed afterwards. The AC3 weighed 29 long tons, 65 mm of cast frontal armor, 45 mm on the sides. Crew of four, reduced from five by deleting the hull machine gun position to make room for 120 rounds of 25-pounder ammunition.
Top road speed of 35 mph, range 200 miles. Turret traverse upgraded to a 110-V electrical system. The most arresting feature was the engine. Australia had no tank engine industry. Pratt & Whitney aircraft radials were committed elsewhere. A pair of GM 6-71 diesels was rejected as inadequate.
The Cadillac Series 75 V8, a 5.7-L side-valve luxury car engine, was readily available. A single one delivered only 135 horsepower, hopelessly inadequate for a 29-ton tank. Watson’s team tripled it. The AC1 used a cloverleaf arrangement, two engines abreast, a third behind, each with its own drive shaft and radiator, all feeding a common transfer box.
It worked. 330 horsepower, but the rear engine suffered chronic cooling problems, and the transfer box bled mechanical losses. Pasholok calls it the Achilles’ heel of the Sentinel. For the AC3, French engineer Robert Perrier, who’d escaped to Australia from Japan in 1941 and brought Hotchkiss design experience with him, developed something elegant.
All three V8s mounted on a single welded steel monoblock crankcase in a triangular delta, one engine on top, two below at 60° to either side. A common combining gear took the three crankshafts into a single output shaft. Each engine could be decoupled individually. One radiator replaced three. The Perrier Cadillac 41-75, 24 cylinders, 397 horsepower, 17 L of displacement.
Max Motor City Garage describes it as a WW24. The flatter engine deck allowed extra fuel tanks. Cooling was solved. Tank Encyclopedia notes the Perrier Cadillac performed significantly better than the earlier engine in terms of torque and horsepower, particularly at lower rpm. The suspension came from a different country again.
The horizontal volute spring system was lifted from the French Hotchkiss H35 and H39, almost certainly via Perrier’s pre-war work at Hotchkiss. The Australian War Memorial’s curatorial record is explicit that the suspension was copied from the French Hotchkiss design of the 1930s. It is unrelated to American HVSS despite the similar name and predates American HVSS Shermans by years.
By February 1943, the AC3 pilot model passed evaluation. The Director of Artillery’s report on turret-mounted firing recorded one of the more memorable observations of any wartime tank trial. Accuracy, the report stated, was superior to that of a field mounting. Noise and blast within the turret was minimal, similar to that of a big air rifle.
Once laid, successive shots could be fired on target without needing to relay the gun. Quick moment here. If you’re enjoying this deep dive into a corner of armored warfare most channels skip, a like genuinely helps. Now, back to the test that everyone remembers, the one with two guns. The tank that fired two 25-pounders was not in fact an AC3.
This is the single most important correction to the popular record. The vehicle was the ACE1, the very first AC1 development hull, made of multi-piece mild steel rather than cast armor, modified in October 1942 with a new prototype turret whose ring had been enlarged from 54 to 64 in. The new turret was purpose-built to host a 17-pounder.
The twin 25-pounders were a placeholder. No 17-pounder existed in Australia. Britain had only just begun 17-pounder production in early 1942. Every gun was committed to eighth Army anti-tank batteries in North Africa. The Maribyrnong Ordnance Factory’s first locally manufactured 17-pounders did not emerge until October and November 1942.
Watson and the Directorate of AFV production needed to validate the new turret structure, the mantlet, the ring, and the hull mountings before committing to AC4 production tooling. So, they improvised. Two 25-pounder gun howitzers were mounted side by side in a single mantlet on a shared recoil system.
Firing one fired both. The arrangement filled the majority of the turret interior. There was no room for crew during firing. The guns were fired remotely by lanyard running out the rear of the turret. Tank Encyclopedia and Secret Projects Forum both cite the same engineering figure.
Simultaneous firing of two 25-pounders produced approximately 120% of a single 17-pounder’s recoil force, a deliberate over-simulation building a roughly 20% safety margin into the test, the 17-pounder’s recoil stroke was around 40 in, which the Australian over barrel recuperator was designed to compress. The trial took place at Fort Gellibrand at Williamstown near Melbourne.
Tank Encyclopedia dates the trials to October 1942. A specific firing date of November 2, 1942 is recorded for the twin 25 configuration. Both are likely correct, indicating preliminary October trials followed by the formally recorded firing. The result against expectations of breathless drama was an engineering success without anecdote.
The tank survived intact. No primary source describes structural failure, cracking of the turret, dislodgement, or damage to the turret ring or hull. The simultaneous discharge produced its expected 120% of 17-pounder recoil, and the structure absorbed it. A persistent legend claims the twin gun firing blew the turret off or injured observers.
This is not supported by any primary source. The lanyard firing was a precaution, not evidence the test went wrong. The Australian War Memorial’s curatorial record, Mellor’s official history, and Hopkins’ 1978 account all describe the trial as a routine engineering success. The dramatic version appears to be internet folklore, conflating fired remotely from outside with something went wrong.
With the structural test passed, the actual 17-pounder, one of the first manufactured at Maribyrnong, was installed in the same turret on the same AC1 hull. The first 17-pounder firing took place on November 11th, 1942. This made the AC1 one of the very first Allied tanks ever to mount and fire a 17-pounder gun.
It predated British Challenger trials. It predated the Sherman Firefly. How does this compare to what else was rolling around battlefields in the same window of the war? The AC3’s 25-pounder armor-piercing shot achieved roughly 63 mm of penetration at 500 m and 58 at 1,000. The American Sherman 75 mm to M3 managed 60 to 76 at 500.
The German Panzer 4 with the long-barreled L/43 gun reached around 91 at 500. The Panzer 4 with the L/48, around 104. The British Cromwell carried similar guns to the Sherman. None of these came close to the 17-pounder’s intended AC4 performance, which projected 130 to 163 mm at 500 m and 119 to 150 at 1,000.
But, this comparison sells the 25-pounder short on a different metric. Its high explosive shell, with 5.2 kg of high explosive in an 11.4 kg projectile, was substantially more lethal than the Sherman 75 M3 HE. For the Pacific theater’s actual mission, bunkers, troops, light Japanese armor, the Sentinel’s gun was arguably better suited than what the Sherman carried.
The Sentinel program ended in July 1943, not because the engineering failed. Lend-Lease worked. American M3 Lee tanks were arriving in Australia in volume, 757 by December 1942. British Matildas were available from mid-1942. Australian skilled labor was needed for landing craft, locomotives, and aircraft.
By mid-1943, the threat of Japanese invasion had passed. The Pacific war required jungle infantry support, not cruiser tanks slugging it out with Panzers. The Australian War Memorial’s curatorial note on AC3 serial 8066 puts it elegantly. With the supply of American and British tanks exceeding requirements, the AC series became unnecessary and an extravagant use of Australia’s limited resources.
A planned AC4 would have weighed 30 long tons, mounted the 17-pounder with up to 74 rounds, used a redesigned 70-in turret ring, and incorporated an 18-round mechanical magazine for ammunition. Director of AFV production AR Code’s report from July 21, 1943, dated within days of cancellation, described the magazine in detail.
None of it was built. The legacy is small, but specific. One Sentinel AC3 exists, serial 8066, the only Thunderbolt ever completed, held at the Australian War Memorial’s Treloar Technology Centre in Canberra. Two AC1 survive, one running at the Royal Australian Armoured Corps Memorial at Puckapunyal in Victoria, and one at the Bovington Tank Museum in Dorset.
The AC1, the twin gun test hull, was traced to the Melbourne Tank Museum until its closure in 2006. According to Tank Encyclopedia, its eventual fate is not known. Almost certainly scrapped. But, Watson returned to Britain in 1943 carrying photographs and drawings of the Australian 17-pounder mounting.
He met Sir Claude Gibb, the Australian-born Director-General of AFVs. According to Tank Encyclopedia’s Australian sources, the over barrel short recoil recuperator system the Australians developed for the AC1 17-pounder shows up in the Sherman Firefly’s mounting design. The precise extent of the technology transfer remains a subject for further archival research, but the connection is not nothing.
October 1942, a tank with two cannons, a lanyard pulled from outside the turret, two guns firing as one, generating 120% of the recoil of a gun Australia did not yet possess. The turret held. That afternoon at Fort Gellibrand was not a stunt. It was a country with no tank tradition, given 2 years in a railway workshop, building the world’s first single-piece cast tank hull in a nickel-free alloy, then mounting an artillery piece in its turret, then mounting two artillery pieces in a successor turret, then mounting an actual 17-pounder before most British tanks had one. The AC4 never fought. It never needed to. The Sentinel made itself unnecessary by helping to demonstrate that an Allied victory was now an industrial certainty. Two guns fired at Fort Gellibrand on a Wednesday in 1942 and proved the tank could survive a gun it would never carry.
